TNF (tumor necrosis factor) family ligands and their corresponding receptors (TNFR) play pivotal roles in mammalian cell host defense processes, inflammation, apoptosis, autoimmunity, and organogenesis. There are at least 18 TNF ligands and 27 receptors identified so far. Some ligands have multiple receptors, and some receptors also bind multiple ligands. The interactions between ligands and receptors are usually very specific and have high apparent affinity (0.1 nM-1 nM) (Locksley et al., 2001, Cell 104:487-501; Fesik et al., 2000, Cell 103:273-282).
The first TNF ligand trimer structure (TNFα) was determined more than a decade ago (Jones et al., 1989, Nature 338:225-228; Eck et al., 1989, J. Biol. Chem. 264:17595-17605). It consists entirely of β strands and loops. The structure has a standard ‘jellyroll’ topology and is remarkably similar to capsid proteins of small RNA viruses such as satellite tobacco necrosis virus (Jones et al., 1984, J Mol Biol 177: 735-767). Three monomers of TNFα form a trimer through highly conserved hydrophobic surfaces. The trimer also exists in solution. Structures of TNFβ, CD40L, and TRAIL were subsequently determined (Eck et al., 1992, J. Biol. Chem. 267:2119-2122; Karpusas et al., 1995, Structure 3:1031-1039; Cha et al., 1999, Immunity 11:253-261). These structures are similar to the TNFα structure, although to the sequence homology is low (20-25%) among the TNF family members. These studies led to proposals that all TNF family members have similar structures and function as trimers (Locksley et al., 2001, supra; Fesik, 2000, supra). Due to the scarcity of available structures (4 of 18) and low sequence homology among the TNF family members, the generality of this conclusion is unclear.
The structure of the complex of TNFβ and cysteine-rich domains (CRDs) from its cognate receptor, TNFR1, has also been determined (Banner et al., 1993, Cell 73:431-445). The structure showed that the three elongated receptor domains bind to one TNF trimer at the interfaces formed between the TNF monomers. Two CRDs (CRD2 and CRD3) make contacts with two distinct regions of TNFβ. The recently determined complex structure of TRAIL and DR5 disclosed a similar interaction mode as observed in the TNFβ and TNFR1 co-crystal structure, although CRD3 of DR5 assumes a different orientation compared to the one in the TNFβ and TNFR1 structure (Mongkolsapaya et al., 1999, Nat. Struct. Biol. 6:1048-1053; Hymowitz et al., 1999, Mol. Cell 4:563-571). It was proposed that the TNF trimeric ligands trigger the trimerization of their cognate receptors, which causes the cytoplasmic regions of the receptor to form a cluster that can recruit adaptor proteins, leading to the activation of downstream signal transduction pathways (Fesik, 2000, supra; Banner, et al., 1993, supra; Mongkolsapaya, et al., 1999, supra; Hymowitz, et al., 1999, supra). This theory is now challenged by new findings showing that TNF receptor and Fas exist in an oligomeric state through the pre-ligand-binding assembly domain (PLAD) before the binding of ligands (Chan et al., 2000, Science 288:2351-2354; Siegel et al., 2000, Nat. Immunol. 1:469-474).
TALL-1, also known as, BAFF, THANK, BLyS and zTNF4, and its receptors BCMA, BAFF-R and TACI are four recently identified TNF/TNFR (TNF receptor) family members (Shu et al., 1999, J. Leukocyte Biology 65:680-683; Schneider et al., 1999, J Exp Med. 189:1747-56; Moore et al., 1999, Science 285:260-263; Mukhopadhyay et al., 1999, supra; Shu et al., 2000, Pro. Natl. Acad. Sci. USA 97:9156-9161; Gross et al., 2000, Nature 404:995-999; Thompson et al., 2000, J Exp Med 192:129-35; Marsters et al., 2000, Curr Biol. 10:785-8; Xia et al., 2000, J Exp Med. 192:137-43; Yan et al., 2000, Nat. Immunol. 1:37-41; Yu et al., 2000, Nat Immunol. 1:252-6; Thompson et al., 2001, Science 293: 2108-2111; and Yan et al., 2001, Curr Biol. 11(19): 1547-52). Overexpression of sTALL-1 in mice leads to increased numbers of mature B-lymphocytes, splenomegaly, anti-DNA antibodies, proteinuria, and glomerulonephritis. These phenotypes mimic those of systemic lupus erythematosus (Shu et al., 2000, supra; Gross et al., 2000, supra; Thompson et al., 2000, supra; Marsters et al., 2000, supra; Xia et al., 2000, supra; Yan et al., 2000, supra; Mackay et al., 1999, J Exp Med 190:1697-710; Khare et al., 2000, Proc Natl Acad Sci USA 97:3370-5). The experiments of BAFF knock-out showed that BAFF was absolutely required for normal B cell development (Schiemann et al., 2001, Science 293:2111-2114; Gross et al., 2001, Immunity 15(2):289-302). The phenotype is similar to that caused by BAFF-R deficiency (Thompson et al., 2001, supra; Yan et al., 2001, supra). In the other hand, the knock-outs of BCMA and TACI did not lead to any severe B cell phenotypes (Xu et al., 2001, Mol. Cell. Biol 21:4067-4074; Von Bulow et al., 2001, Immunity 14:573-582). Interestingly, APRIL (also called TALL-2), the closest family member of TALL-1, does not bind to BAFF-R (Schiemann et al., 2001, supra), although it binds to BCMA and TACI with an affinity similar to sTALL-1 (Yu et al., 2000, supra).
In contrast to the other receptor family members that have at least three to four CRDs in their extra-cellular domains, BCMA and BAFF-R have only one CRD and TACI has two CRDs (Shu et al., 2000, supra; Gross et al., 2000, supra; Thompson et al., 2000, supra; Marsters et al., 2000, supra; Thompson et al., 2001, supra). Nevertheless, the overall binding affinities of sTALL-1 with BCMA and TACI (0.1 nM-1 nM) are similar to those of other family members (Yu et al., 2000, supra). Furthermore, as predicted from sequence alignment, the CRDs in BCMA and TACI contain A1 and C2 modules (Gross et al., (2000) Nature, 404:995-999), which were two of multiple defined structural motifs that characterize extracellular domains of TNF receptors (Naismith et al., 1998, TRENDS Biochem. Sci. 23:74-79). The C2 module was also found in TNF-R1 and Fn14 (Bodmer et al., (2002) J Biol Chem., 275:20632-20637), however the C2 in TNF-R1 is not involved in ligand binding (Naismith et al., 1998, supra). The only CRD in BAFF-R that was predicted to be the C2 module initially (Thompson et al., 2001, supra; Yan et al., 2001, supra) has been termed an unknown module X2 recently (Bodmer et al., 2002, supra). It is likely that there are novel interactions among these unique ligand-receptor couples accounting for their high affinity (Liu et al., 2002, Cell 108:383-394; Bodmer et al., 2002, supra). Therefore, to begin to understand the structure and function relationship of TALL-1 and its receptors, and to take advantage of this information to design valuable therapeutic tools, it is necessary to determine the crystal structure of sTALL-1 and its receptors.